Star Phenomena Resulting From Interference of Light

If two rays of identical light are superimposed so that they travel
in exactly the same path, two things may occur: (1) If they are in phase,
they will ASSIST each other; (2) if they are out of phase, they will
tend to CANCEL each other. In the latter case, if they are exactly out
of phase, the wave motion will be entirely cancelled, resulting in DARKNESS.
When they assist each other, the result is INCREASED INTENSITY. If rays
of WHITE light overlap, the resultant interference may produce spectral
colors. In other words, waves of certain of the hues that comprise white
light may cancel each other, leaving only waves of certain other hues.

In figure 4 two waves (A and B) of the same length traveling the
phase XY at the same time, but so timed that A is on the upward swing
when B is on the downward motion, result in the two cancelling each
other. In effect, A and B, which are equal in length, are subtracted
from one another.

When the two waves are superimposed on one another so that each is
moving from X to Y "in step" (as in Figure 5), the result is intensification;
A is added to B. If two waves are between the two extremes shown in
Figures 4 and 5, the results vary from some cancellation to some intensification
of the light, depending on whether the relationship of A and B is closer
to that illustrated in Figure 4 or than in Figure 5.

There are only a few situations in nature in which the necessary
conditions for interference occur, for the basic condition of two or
more beams travelling the same path at the same time is an unusual occurrence.

Figure 6 illustrates how interference is produced. A ray of light
is directed at an extremely-thin transparent film such as oil on water.
To simplify the illustration, we will consider only two portions of
the ray, labeled A and B. As these rays strike the film, a portion of
the light is reflected; the remainder is refracted into the film, reflected
from the bottom surface, and refracted back into the air, following
the same path as the light reflected from the surface. This point at
which the reflected and refracted rays coincide and begin to travel
the same path is encircled in the sketch.

Thus for a certain thickness of the layer, or film, as well as of
different angles of incidence for the light, the red waves might amplify
o ne another to produce an intense red light, whereas some of the other
waves are cancelled; or the red rays may cancel one another and the
blue rays be intensified. The colors thus produced are known as INTERFERENCE
COLORS. The brilliant colors of soap bubbles and the colors produced
by cracks in topaz, rock crystal, glass, etc. are due to this interference.
Whenever there are such thin, transparent films or layers of different
material, this interference of light occurs, with its resulting colors.

Play of Color

This is the name applied to the multiple colors displayed by fine
opals. These colors form a patchwork, with the color of each patch changing
as the stone is turned. A predominance of red flashes is most desirable,
with green next, and blue less desirable.

Play of color is caused by diffraction of light and variations in
refractive index from innumerable, regularly arranged, optically transparent
spherical particles of amorphous silica and from the spaces, or voids,
between these particles. The spheres, and hence the voids, are arranged
regularly in three dimensions (face-centered cubic), so that the whole
arrangement makes a three-dimensional diffraction grating. The important
feature is that the spacing of the voids is the same as that of the
spheres, and when this is about that of the wavelength of visible light,
diffraction occurs. The angle through which the light is diffracted
varies continuously with wavelength, so that different colors appear
at different angles, thus producing play of color. Only pure spectral
colors can arise from this process. This theory of the internal structure
of precious opal was proved in the mid 1960's by research with the electron
microscope.

The play of color associated with fine opal is sometimes referred
to as "opalescence" but this term more correctly refers to an internal
milky or pearly appearance. Because of its double usage, the word is
avoided in the gemology. Play of color should not be confused with "fire",
which is another name for dispersion, as observed in transparent faceted
stones.

Labradorescence (pronounced lab-rah-door-ESS-ence)

Labradorite, a species of the feldspar group, is usually a some what
dull gray stone in the rough. When it is polished, large patches of
a vivid solid hue often appear that change gradually as the stone is
moved. They do NOT appear in flashes as in the opal. This effect is
sometimes called CHANGE OF COLOR, as distinguished from the play of
color in opal. However, as mentioned previously, change of color is
also used to describe the phenomenon seen in alexandrite, an entirely
different effect.

Labradorite is repeatedly twinned and is made up of a large number
of very thin plates. These thin plates set up the condition necessary
for light interference, as described in the foregoing paragraphs. In
addition to this effect, it is probable that a distribution of tiny
areas of feldspar of a different composition cause a diffusion, or scattering,
of light that is superimposed on the interference caused by the thin
plates.

Iridescence (pronounced ear-ih-DESS-ence)

Iridescence is a display of prismatic colors produced by light interference
from very narrow fissures enclosing thin films of air or liquid. Such
a condition is given a name in quartz (iris quartz or iris agate), but
it may be encountered in any gemstone that has been fractured or in
which a cleavage has started to develop.

Orient

This is the prismatic sheen on which the beauty of the pearl mainly
depends. Interference and diffraction of light on the tiny overlapping
plates on the surface of the pearl produces this effect. Diffraction
produces the multiple colors sometimes seen on the jagged edges of broken
glass. It is a phenomenon distinct from both dispersion and play of
color.

Adularescence (pronounced ad-u-lar-ESS-ence)

Adularescence or moonstone effect, often confusingly called "opalescence"
or "schiller" is the name given to feldspar (usually orthoclase) that
exhibits a blue to white sheen in certain directions. When the stone
is cut in cabochon, a floating, billowy, bluish or white light is noticed
as the stone is turned.

This moonstone effect, which is visible only in certain directions
in the feldspar crystal, seems to be caused by somewhat diffused reflection
of light from repeated-twinning planes or parallel intergrowths of another
feldspar of a slightly different R.I. from the main mass of orthoclase.

Girasol Effects (pronounced JIR-ah-sol)

The adjective girasol is applied to the varieties of several species
the minerals, including opal, corundum, chrysoberyl and quartz, that
exhibit a movable or billowy light effect as the stone is turned. The
effect is often mistaken for adularescence, but, although somewhat similar,
the appearance is more cloudy. As a noun, girasol refers only to the
variety of opal that produces this effect, but it is used as an adjective
in such terms as girasol sapphire .and girasol quartz.